The present invention relates to an image forming apparatus of the intermediate transfer method, and more particularly, to an image forming apparatus including a pre-secondary-transfer charging section and a concentration detection section.
An image forming apparatus of the intermediate transfer method forms a color toner image as follows. Toner images are formed on photoreceptor surfaces for hues of yellow, magenta, cyan, and black. An intermediate transfer belt is rotated while transferring the toner images sequentially onto the belt and overlapping the images thereon at a primary transfer position at which the photoreceptor and the intermediate transfer belt contact each other. The image forming apparatus then transfers the color toner image formed on the intermediate transfer belt to a sheet at a secondary transfer position at which the intermediate transfer belt and the sheet contact each other.
Some conventional image forming apparatuses, as described by Japanese published unexamined application No. 2003-57959 for example, improve the transferability at the secondary transfer position by providing, in the traveling direction of the intermediate transfer belt, a pre-secondary-transfer charger upstream of the secondary transfer position, and applying to the toner images on the intermediate transfer belt a charge of the same polarity as the toner charge polarity. These image forming apparatuses apply a uniform charge from the pre-secondary-transfer charger to the toner image by reducing oscillation of the intermediate transfer belt by providing a first roller at a position opposed to the pre-secondary-transfer charger across the intermediate transfer belt.
Other conventional image forming apparatuses, as described by Japanese published unexamined application No. 2008-241958 for example, improve the image quality by optically sensing the toner concentration of the toner image pattern on the intermediate transfer belt with a toner concentration sensor, and controlling the toner concentration depending on the sensing results. These image forming apparatuses improve the toner image detection accuracy by reducing oscillation of the intermediate transfer belt by providing a second roller at a position opposed to the toner concentration sensor across the intermediate transfer belt.
The combination of the ideas described by Japanese published unexamined applications No. 2003-57959 and No. 2008-241958 may provide the image forming apparatus with the pre-secondary-transfer charger and the toner concentration sensor to improve the transferability and to improve the image quality.
Unfortunately, in the image forming apparatus, the first roller at the position opposed to the pre-secondary-transfer charger and the second roller at the position opposed to the toner concentration sensor, as described above, can cause more rollers to rub the intermediate transfer belt, thereby facilitating the wear of the intermediate transfer belt and reducing the durability thereof.
Absence of the first and second rollers can cause oscillation of the intermediate transfer belt during the rotation. The charge applied to the toner image by the pre-secondary-transfer charger thus varies and the toner concentration sensor detects the toner image pattern at reduced accuracy, thereby reducing the image quality.
In view thereof, it is an object of the invention to provide an image forming apparatus that reduces the wear of the intermediate transfer belt to improve the durability thereof. It is another object of the present invention to provide an image forming apparatus that improves the transferability to provide a higher quality image.